There have been increasing efforts in recent years to save energy to prevent global warming, which is a global environmental issue. For example, in the case of thermal devices such as refrigerators and vending machines, it used to be common to inject closed-cell urethane foam into the insulation space between the outer and inner boxes composing the insulation box of these thermal devices. Recently, however, it is more common to provide a vacuum insulation material into the insulation space and to inject closed-cell urethane foam into the space.
A vacuum insulation material is composed of a bag-shaped gas-barrier covering material and a core made of glass wool or other similar material and sealed in the covering material under reduced pressure. Some vacuum insulation materials include a core and an adsorbent both of which are sealed under reduced pressure. Vacuum insulation materials have about 20 times better insulation performance than rigid urethane foam, and their insulation performance can be maintained even if the materials are smaller in thickness.
For these reasons, vacuum insulation materials are attracting attention as an effective means to meet customers' demand for larger-volume insulation boxes as well as to increase insulation performance to achieve energy saving.
However, the insulation spaces of insulation boxes such as refrigerators tend to have complicated shapes. This sets a limit to the increase in the covering area of a vacuum insulation material, or in other words, in the area proportion of the vacuum insulation material in the total heat transfer area of the insulation box.
To address this issue, PTL 1 proposes the following technique which does not employ vacuum insulation material. Open-cell urethane foam with a porous structure is injected into the insulation space of an insulation box first, and then the insulation box is vacuum-evacuated by a vacuum-evacuator connected to the outlet of the box. An open-cell structure is a structure where the cells are communicated with each other. In contrast, a closed-cell structure is a structure where the cells are isolated from and not communicated with each other.
When the insulation space between the outer and inner boxes of an insulation box such as a refrigerator is wholly formed into a vacuum insulation layer without using a vacuum insulation material, open-cell urethane foam is used. If closed-cell urethane foam is used instead of open-cell urethane foam, residual gas in the closed cells comes out after vacuum evacuation, causing a decrease in the degree of vacuum.
The open-cell urethane foam does not merely have an open-cell structure. It is also to have air permeability between adjacent cells both in cell film portions (membranous portions between the cells) of the urethane foam and in cell framework portions (connected to the cell film portions and thicker than these). The reason for the need of air permeability between adjacent cells is as follows. In regions with a relatively low density of cells and a high content of the resin of the open-cell urethane foam (many of the cell framework portions), it is difficult to form through-holes in the resin. As a result, the resin blocks the communication between the cells, failing to completely vacuum-evacuate the insulation space.
Open-cell urethane foam is to have air permeability between adjacent cells not only in a core layer at the center but also in a skin layer at and near the boundaries between the outer and inner boxes because the skin layer includes more of the cell framework portions than the core layer.
For example, in the open-cell urethane foam shown in PTL 1 having air permeability between adjacent cells both in the core layer and in the skin layer, the insulation space can be vacuum-evacuated.
However, the through-holes in each cell film portion and cell framework portion of the above open-cell urethane foam are as small as several microns in diameter. This causes the urethane foam as a whole to have a high evacuation resistance. As a result, it takes several hours to several days to vacuum-evacuate the insulation space of a large container such as the door or housing of a refrigerator, and it is far from commercialization.